1. Field of the Invention
This invention relates to the field of electro-surgical medical devices. More particularly, this invention relates to devices that deliver energy in the form of radio-frequency electrical current to tissue in order to perform surgical functions.
2. Description of Related Art
Various medical procedures rely on high-frequency electrical currents to deposit energy and thus heat human and animal tissues. During such procedures, a high-frequency current is passed through the tissue between electrodes. One electrode is located at the tip of a surgical probe. Another electrode is located elsewhere, and may be a ground pad or another surgical probe tip. The tissue to be treated lies between the electrodes.
When the electrode circuit is energized, the electric potential of the electrodes at the probe tips oscillates at radio frequencies about a reference potential. If one is used, a ground pad remains at a floating reference potential. As the electric potential of the probe electrodes varies, a motive force on charged particles in the tissue is established that is proportional to the gradient of the electric potential. This electromotive force causes a net flow of electric charge, a current, to flow from one electrode, through the tissue, to any other electrode(s) at a lower potential. In the course of their flow, the charged particles collide with tissue molecules and atoms. This process acts to convert electrical energy to sensible heat in the tissue and is termed Joule heating.
Upon heating, surgical functions such as cutting, cauterizing and tissue destruction can be accomplished. For example, tissues can be cut by heating and eventually vaporizing the tissue cell fluids. The vaporization causes the cell walls to rupture and the tissue to cleave. When it is beneficial to destroy tissue, comparatively higher rates of energy deposition can cause tissue ablation.
Ablation of cellular tissues in situ is used in the treatment of many diseases and medical conditions either alone or combined with surgical removal procedures. Surgical ablation is often less traumatic than surgical removal procedures and may be the only alternative where other procedures are unsafe.
Tissue ablation devices commonly utilize electromagnetic (microwave, radio frequency (RF), lasers) or mechanical (acoustic) energy. In the category of electro-surgical devices, microwave ablation systems utilize a microwave antenna which is inserted into a natural body opening through a duct to the zone of treatment. Electromagnetic energy then radiates from the antenna through the duct wall into the target tissue. However, there is often severe trauma to the duct wall in this procedure since there is a significant microwave energy flux in the vicinity of the intended target. The energy deposition is not sufficiently localized. To reduce this trauma, many microwave ablation devices use a cooling system. However, such a cooling system complicates the device and makes it bulky. Laser ablation devices also suffer the same drawback as microwave systems. The energy flux near the target site, while insufficient to ablate the tissue, is sufficient to cause trauma.
Application of RF electric currents emanating from electrode tips offers the advantage of greater localization of the energy deposition since the electrode tip is nearly a point source. However, these devices require consideration and accurate monitoring of the time rate of energy transfer to the tissue. Since the electric energy flux is localized, the electrical dissipation and storage characteristics of the tissue carrying the current may vary with time as a result of the current-induced heating. Thus, the power absorbed by the tissue could vary over the time of treatment due to changing values of the tissue's electrical properties.
The localization of energy flux in an RF electro-surgical device may require a number of electrodes to be included in the surgical probe to provide adequate area coverage. This may result in the electric power being delivered across several current paths. With multiple electrodes in a surgical probe, each probe electrode may or may not be at the same electric potential at each instant due to amplitude, frequency, or phase variations in their RF oscillations. If each probe electrode is at the same potential, then a current will flow between the probe electrode and the ground pad. This mode of operation is termed monopolar. If, however, each probe electrode is not an identical potential, current will flow between the probe electrodes. This mode of operation is termed multipolar. If there are potential differences between the probe electrodes and there is a ground pad, then there are currents between the probe electrodes as well as currents between the probe electrodes and the grounding pad. This model of operation is a combination of monopolar and multipolar modes. It is noteworthy that in the case of multipolar operation, the probe electrodes are electrically coupled by the currents flowing between them. The extent of the coupling is primarily determined by the difference in electric potential between the probe electrodes and the electrical properties of the tissue between the electrodes. This coupling can confuse monitoring of applied power and tissue response.
Power control is critical in an RF electro-surgical device since it is directly related to the intended medical effects. As described, the power absorbed by the tissue can vary over the time of treatment due to changing values of the tissue's electrical properties. This variation is due to a relation well-known to those skilled in the art in which the instantaneous power delivered to the tissue load is proportional to the square of the electrode voltage and inversely proportional to the tissue electrical impedance. Thus, to achieve equal power delivery, two surgical probe electrodes may have to be at different electric potentials (voltages) because of Joule heating effects on the tissue electrical impedance, or because of impedance gradients in the tissue. When the surgical probe electrodes are at different electric potentials, a current will flow between the electrodes. This electrode coupling is commonly referred to as cross-talk. Cross-talk confuses accurate power determination in most RF electro-surgical devices and thus there is a need for improved methods to control power delivery.